Multimodal Distributions along the Horizontal Branch

We report on HST/WFPC2 U,V and far-ultraviolet observations of two GalacticGlobular Clusters (GGCs), NGC 6205 = M13 and NGC 6093 = M80. Both of theseclusters have horizontal-branch (HB) tails that extend to the helium-burningmain sequence, with the hottest stars reaching theoretical effectivetemperatures above 35,000 K. In both clusters, groups of stars are found to beseparated by narrow gaps along the blue HB sequence. These gaps appear atsimilar locations in the color-magnitude diagrams of the two clusters. Whilestochastic effects may give rise to variations in the color distribution alongthe HB, the coincidence of gaps in different clusters effectively rules thisout as the primary cause. The comparison among the clusters strongly suggeststhat there are separate physical processes operating during the earlierred-giant phase of evolution to produce mass loss.